As methods for producing clad-steels with a multi-layered structure, there have been known an internal chill method of casting, an explosion bonding method, a roll-bonding method, a cladding method by welding and so on. More specifically, a surface layer of the clad-steel is formed of expensive austenitic stainless steel and an inner layer of the clad steel is formed of cheap normal steel, so that the clad steel product has characteristics of stainless steel and is advantageous in that it can be manufactured more inexpensively than steel materials entirely formed of the austenitic stainless steel.
A continuous casting method of a multi-layered slab as the clad steel has already publicly been known as the prior art previously proposed by the present inventors (refer to JP-A-63-108947). The casting method aims to obtain a multi-layered slab by solidifying two kinds of molten metals which are a content poured in a continuous casting mold while separating the molten metals by magnetic means. In this method, direct current magnetic flux is given at a location of a certain height of the mold, extending transversely to the materials in the mold, and the molten metals having different compositions are respectively supplied above and below a boundary of static magnetic fields formed by the direct current magnetic flux, thereby obtaining a composite metallic mass having the previously solidified upper material (which becomes a surface layer of the solidified casting slab) and the successively solidified lower material (which becomes an inner layer of the solidified casting slab); a boundary between the upper and lower portions of the content is clearly defined, that is to say, the concentration transition layer between the surface layer and the inner layer is thin.
The continuous casting method of the above-described multi-layered slab will now be explained more particularly with reference to FIGS. 3 and 4.
Direct current magnetic flux is applied to a content 4 (molten metals) poured in a continuous casting mold 1 in a molten state, the direct current magnetic flux extending transversely in a direction of thickness of the content over the entirety width of the materials (numeral 10 designates a line of magnetic force). Two kinds of molten metals having different compositions which are the content, are supplied through refractory dip nozzles 2 and 3 above and below a boundary of static magnetic fields 11 formed by the direct current magnetic flux longitudinally in a casting direction. In FIG. 4, it is a cross-sectional view of casting slab 9 to be manufactured, there are shown a solidified surface layer and a solidified inner layer 6. The direct current magnetic flux is formed by magnets 8 in a perpendicular direction to the casting direction A, that is, transversely in the direction of thickness of the content or the partially solidified casting slab in the mold.
It has been recognized from the investigation by the inventors of this application that the publicly-known continuous casting method has a problem that convection mixing resulted from a difference in density between the molten steels in the mold, sometimes happens when a combination of the steels is inadequate so that a mixing restrain effect against the molten steels is not fulfilled by the direct current magnetic flux and preferable separation between the two kinds of molten steels cannot be obtained.